Nowadays, vehicle door handles are equipped with devices for detecting the presence of a user. The detection of the presence of a user coupled with the “hands-free” recognition of an electronic badge for remote access control, said badge being carried by this user, enables the remote locking and unlocking of the openings of the vehicle. Thus, when the user, who is carrying the corresponding electronic badge, which has also been identified by the vehicle, wishes to unlock the vehicle, he approaches or touches the door handle of the vehicle. The openings of the vehicle are then unlocked automatically. By approaching or pressing on a specific part of the door handle of the vehicle, referred to as the “unlocking zone”, the door (or alternatively all the openings) is (are) unlocked without further action on the part of the user. Conversely, when the user, still carrying the necessary badge, which has also been identified by the vehicle, wishes to unlock his vehicle, he closes the door of his vehicle and approaches or momentarily presses on another specific part of the handle, referred to as the “locking zone”. This gesture makes it possible to automatically lock the openings of the vehicle.
These presence detection devices generally comprise two capacitive sensors, in the form of two electrodes electrically connected to a printed circuit board, integrated in the door handle, in each case in a specific locking or unlocking zone. Generally, one electrode is dedicated to each zone, that is to say one electrode is dedicated to the detection of the approach and/or of the contact of the hand of the user in the locking zone and one electrode is dedicated to the detection of the approach and/or of the contact of the hand of the user in the unlocking zone.
The presence detection device further comprises a radio frequency antenna, generally LF (low frequency). The detection device is connected to the electronic control unit (ECU) of the vehicle and sends a presence detection signal to said unit. The electronic control unit of the vehicle has identified the user beforehand as being authorized to access this vehicle, or alternatively proceeds with this identification following the receipt of this presence detection signal. To this end, it sends an identification request by means of the radio frequency antenna to the badge (or to the remote control) carried by the user. In response, this badge sends its identification code via RF (radio frequency) waves to the electronic control unit of the vehicle. If the electronic control unit recognizes the identification code as that authorizing access to the vehicle, it triggers the locking/unlocking of the door (or of all the openings). If, by contrast, the electronic control unit has not received an identification code or if the received identification code is incorrect, the locking or unlocking is not performed.
Such vehicles are thus equipped with door handles comprising a detection device itself comprising a radio frequency antenna, generally low frequency, or at least one electrode connected to a printed circuit board.
Merely by way of explanation, a detection device D comprising just a single electrode, the electrode being dedicated for example to the unlocking zone connected to a printed circuit board, and an LF antenna will be considered here. A detection device D of the prior art is described with reference to FIGS. 1 and 2.
FIG. 1 shows a motor vehicle door handle 10 (vehicle not shown), in which a device D for detecting the presence of a user is located. Said door handle 10 comprises a first outer surface S1 oriented in the direction of the door P and a second outer surface S2 opposite the first outer surface S1 and therefore oriented on the side opposite the vehicle, more specifically toward the user (not shown). This detection device D comprises an electrode 12 (unlocking electrode), of which a face is located in the proximity of the first outer surface S1, and control means 13 and an LF antenna 11, of which a face is located in the proximity of the second outer surface S2. The electrode 12 is connected to the control means 13. These control means 13 measure the capacitance at the terminals of the electrode 12 so as to detect the presence of a user and are formed for example by a printed circuit board comprising a microcontroller (not shown). The LF antenna 11 is connected for its part to an electronic control unit installed on the vehicle (not shown) of the BCM (body controller module) type, which manages the identification requests emitted by said LF antenna 11.
FIG. 2 shows the prior art detection device D in greater detail. The LF antenna 11 is provided in the form of a winding around a ferrite, and the electrode 12 is provided in the form of a metal plate, for example a copper sheet, or in the form of a copper surface printed on the control means 13, that is to say on the printed circuit board.
This detection device D of the prior art has two major disadvantages.
The first disadvantage lies in the proximity between the LF antenna 11 and the electrode 12. This proximity creates eddy currents (shown by the arrow CF) in the electrode 12, which are induced by the activation of the LF antenna 11. These eddy currents in the electrode 12 in turn interfere with the operation of the LF antenna 11 and reduce its emission range whilst increasing the temperature of the electrode 12.
The second disadvantage lies in the form of the electrode 12. Since it is formed by a metal plate or a copper surface printed on a printed circuit board, and since it is supplied with current, the electrode 12 behaves as an RLC circuit, that is to say a circuit having a resistance R, an inductance L, and a capacitance C. The values of the resistance R, of the inductance L and of the capacitance C are dependent on the dimensions of the electrode 12, that is to say the length thereof. Le, the width thereof le and the thickness thereof e (see FIG. 2).
Thus, when the electrode 12 (that is to say the RLC circuit) sustains an oscillation, it may start to resonate.
More specifically, the electrode 12 has a characteristic impedance Z, which is minimal for an oscillation frequency value referred to as a resonance frequency Fr and is defined by:
  Z  =                    R        +                  j          ⁢                                          ⁢          L          ⁢                                          ⁢          w                            j        ⁢                                  ⁢        Cw            with:w=2*π*F where:R: resistance of the electrode 12,L: inductance of the electrode 12,C: capacitance of the electrode 12,j: imaginary part of complex numbers,w: pulse of the oscillation,F: frequency of the oscillation.
The transmittance T in accordance with the frequency F of the electrode 12 is illustrated schematically in FIG. 3. The transmittance T has its maximum at the resonance frequency Fr.
When the electrode 12 senses an oscillation, that is to say a radio frequency wave at this resonance frequency Fr, originating from electromagnetic interference resulting from the external environment, the device D for detecting the presence of a user no longer functions. More specifically, the presence of a user close to the door handle 10 can no longer be detected or is detected incorrectly, which results in an unnecessary discharge of the battery of the vehicle. In fact, in this latter case, the capacitive sensor is unnecessarily involved during the incorrect detection phases, whereas it could be in standby and could therefore consume less energy.